Based on my work experience I hazarded the guess, about one person in twenty has mental illness to the degree that it interferes with their ability to function in a work environment. Almost any boss can tell you that 5% of their workers cause 90% of the problems. I expect any cop will give you similar numbers.
Today I went on-line and this is what I found.
Approximately 4% of Americans experienced serious mental illness in the past year. That number excludes substance abuse which is often a marker for self-medicating for mental illness. Throwing substance abuse into the mix could easily double that number to almost 10%.
One model for mental illness is similar to becoming lost. The stream of evidence passing before our eyes does not match up to our mental models. We rationalize and make excuses until undeniable facts refuse to go away and we freak out. I expect an increase in significant mental illness in the event that our current economic troubles have a hard landing.
Our brain rides around in our body. They rewire to meet various tasks. Insults to our bodies damage our brain's ability to function. Overly specialized wiring is slower to rewire to tasks that are totally foreign to it. IMHO, mental illness can befall a perfectly "normal" person when the stream of evidence accelerates to where we can no longer track. It is chemical.
One in every ten people is a diagnosable whack-job. It could be anybody. They cannot be avoided. They are everywhere. I am not paranoid. I am mathematically literate.
A therapist once told me that the difference between suicide and homicide is vanishingly small. Both involve breaking the taboo against taking human life. One who contemplates suicide is also capable of homicide.
People will do strange things. Watch the hands. Leave yourself two avenues for rapid egress. Rapid changes of speed and direction are your friend.